Arrowroot isn't actually a root, though its name comes from the fact that it was often used as a folk remedy to treat wounds caused by arrows. The powder is made by drying and grinding the rhizome -- the part of the plant that the stem and roots grow from -- of the South American plant Maranta arundinacea. Along with its purported benefits as a wound treatment and remedy for digestive disorders, arrowroot powder is an excellent thickener. It's especially valuable in the kitchen when used to keep pie fillings from being runny, to cocoon fruits in a satiny glaze or to make a silky, clear fruit sauce.
Measure your arrowroot powder into a small bowl. The amount will depend on how much fruit sauce you are trying to thicken. A good rule of thumb is about 1 tablespoon of arrowroot for every 1 cup of fruit sauce.
Pour an equal amount of liquid into the bowl. If you are using 1 tablespoon of arrowroot, add 1 tablespoon of cold water for a clear glaze.
You can also use cream, milk, nondairy milk, a liquor like bourbon, or a liqueur such as almond-flavored amaretto or orange-infused curaçao. These choices will result in a less-clear glaze, so water is best if you want the natural color of your fruit sauce to take center stage.
Whisk the arrowroot and liquid together until there are no visible lumps. This is called making a slurry, usually added at the end of the recipe. It is important to completely dissolve the arrowroot into the liquid to avoid clumpy bits in your fruit sauce.
Bring your fruit sauce to a slow boil. Arrowroot thickens at a lower temperature than flour, so you can relax a little and not worry about the fruit sauce scorching as you thicken it.
Slowly pour the arrowroot slurry into the hot fruit sauce, whisking to thoroughly incorporate it into the sauce. Be very careful to whisk gently so that you don't splash yourself with the hot fruit sauce. Arrowroot slurry doesn't thicken instantly, so you can take your time. Cook and stir for 2 to 3 minutes, or until the sauce has reached the desired thickness.